Electro-optical reconnaissance system with forward motion compensation

ABSTRACT

An electro-optical framing camera forward motion compensation (FMC) reconnaissance system comprising a moving shutter and a full frame focal plane array detector is designed to minimize the variation of image motion from a target scene across the focal plane array. The full frame focal plane array, such as a Charge Coupled Device (CCD), is designed to transfer and add the image from pixel to pixel at a predetermined rate of image motion corresponding to the region exposed by the focal plane shutter. The focal plane shutter aperture and velocity are set to predetermined values coordinated with the available illumination. The CCD image transfer rate is set to minimize the smear effects due to image motion in the region of the scene exposed by the focal plane shutter. This rate is variable with line of sight depression angle, aircraft altitude, and aircraft velocity/altitude ratio. Further, a method of FMC utilizes a comparison of a measured light level to a standard value in order to determine the appropriate exposure time and shutter motion rate. An optimal FMC clocking signal is calculated based on image motion equations incorporated in the processing unit of the reconnaissance system.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of the commonly owned, co-pending PCTApplication No. PCT/US97/19897, filed Nov. 5, 1997 (incorporated byreference herein), which claims the benefit of U.S. application Ser. No.60/030,089, filed Nov. 5, 1996 (incorporated by reference herein).

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to electro-optical reconnaissancesystems whose angular resolution is greater than the product of theexposure time and the angular rate of image motion. The invention is aforward motion compensation (FMC) system that permits full resolutionperformance when the target line-of-sight-angular-rate-exposure-timeproduct is greater than the angular resolution of the system. The systemincludes optics, a mechanical shutter and full frame CCD.

2. Related Art

Aerial reconnaissance systems have undergone a dramatic transition inthe past two decades with the replacement of photographic film byelectro-optic image sensors. With the advent of wafer-scale focal planesthat provide sufficient coverage and resolution, reconnaissance systemsare being designed to utilize electro-optic sensors configured as largeformat area arrays. These electro-optic (“EO”) reconnaissance imagingsystems most often employ charge-coupled devices (“CCDs”) operating inthe visible and near-infrared regions of the electromagnetic spectrum tocapture the image of the target or scene. The ability to operate in areal-time environment and in low ambient light conditions are just a fewof the reasons why electro-optical-based reconnaissance imaging systemsare increasingly replacing film-based reconnaissance systems.

One of the more frequently encountered problems in designing aerialreconnaissance imaging systems is determining the most effective methodof compensating for image smear or blurring. Typically, smearing occurswhen low ambient light conditions prevent an imaging system from usingsufficiently short exposure times, resulting in a blurred image due tothe forward motion of the aircraft. In other words, smearing occurs as aresult of the relative motion between a scene or target to be imaged andthe imaging system. Therefore, in order to prevent the degradation ofthe information contained in a recorded image, an ideal reconnaissanceimaging system must utilize some means of image motion compensation(“IMC”) for image smear.

Different reconnaissance mission operating scenarios can presentdifferent image motions that should be compensated for. The goal of anyimage motion compensation system, of which a forward motion compensation(“FMC”) system is a specific category, is to reduce the image smear thatoccurs when the target line-of-sight-angular velocity is significantlydifferent from the camera angular velocity.

Early reconnaissance systems comprised linear arrays that operated athigh altitudes, thereby minimizing the angular motion effectsproportional to the aircraft velocity/altitude ratio. However, when lowflying mission scenarios are required to avoid detection of thereconnaissance aircraft, forward motion compensation is necessary tomaintain image resolution. Several conventional methods of IMC have beendeveloped to meet these image resolution requirements.

For example, U.S. Pat. No. 4,505,559, issued Mar. 19, 1985 to Prinz,discloses an approach wherein an instantaneous line-of-sight controlsthe motion of the film used to record the image. U.S. Pat. No.4,157,218, issued Jun. 5, 1979 to Gordon et al., also uses a film driveto compensate for the forward motion of the image. Mechanical means areused in U.S. Pat. No. 4,908,705, issued Mar. 13, 1990 to Wight, wherethe imaging array physically moves to reduce the smear.

U.S. Pat. No. 5,155,597 to Lareau et al., issued Oct. 13, 1992,discloses an equation that described the correction for the image motionin the side oblique scenario by transferring the charge in a columnsegmented CCD array at different transfer rates corresponding to thedepression angle.

However, these aforementioned image motion compensation techniques areinadequate to provide for image motion compensation in each of thevarious mission scenarios described above. What is needed is anelectro-optical reconnaissance system that provides adequate imagemotion compensation in forward oblique, side oblique, and verticalorientations. In addition, it is desirable that this reconnaissancesystem be low cost.

SUMMARY OF THE INVENTION

The present invention provides a system and method for the compensationof image motion during reconnaissance missions. According to a firstembodiment of the present invention, the electro-optical reconnaissancesystem includes an imaging focal plane array (FPA), such as acharge-coupled device (CCD), to record a target scene. The focal planearray includes a main format area having a plurality of photo-sensitivecells arranged in rows and columns. The reconnaissance system alsoincludes a shutter having a window (or exposure slit) that moves acrossthe imaging device. In order to compensate for the forward motion of thevehicle, such as an aircraft, the charge in the imaging device istransferred across the device. The rate of charge transfer is uniformacross the focal plane array, but varies in time in accordance with theportion of the target being imaged, where the portion of the targetscene being imaged is defined by the position and width of the shutterslit. The charge transfer rate is varied based on the position of theshutter slit over the imaging device. A camera control electronics unitcontrols the position of the shutter slit and processes target sceneinformation, light levels, and reconnaissance mission requirements inorder to determine to the rate of motion of objects contained in theportion of the target scene viewed by the focal plane array. As aresult, the camera control electronics unit can generate an appropriateclocking signal to perform forward motion compensation (FMC) in avariety of target viewing modes, including forward oblique, sideoblique, and vertical modes of operation.

According to a second embodiment of the present invention, a method forproviding forward motion compensation for the electro-opticalreconnaissance system is utilized in the camera control electronicsunit. First, a light sensor measures the light level of the scene to beimaged by the reconnaissance system. Next, the measured light level iscompared to a predetermined light level value. For example, thepredetermined light level value can correspond to a given solar angleabove the horizon. If the measured light value is greater than thestandard value, an exposure time is determined by comparing the measuredlight level to a primary exposure time look-up table. If the measuredlight value is less than the standard value, the exposure time isdetermined by comparing the measured light level to a low exposure timelook-up table. By determining the proper exposure time, the propershutter slit width and shutter slit speed are determined. Next, aforward motion compensation profile is determined corresponding to theexposure time and mission parameter inputs. For example, the missionparameters can include aircraft velocity, altitude, and camera lookangle. This FMC profile corresponds to the clocking signal that is usedto drive the focal plane array of the reconnaissance system in order toperform FMC.

Further features and advantages of the present invention, as well as thestructure and operation of various embodiments of the present invention,are described in detail below with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the pertinent art to makeand use the invention. In the drawings, like reference numbers indicateidentical or functionally similar elements. Additionally, the left-mostdigit(s) of a reference number identifies the drawing in which thereference number first appears.

FIG. 1 illustrates various image motions due to the forward motion of anaircraft;

FIGS. 2A-D illustrate alternative conventional electro-optical imagingmodes;

FIG. 3 illustrates the shifting of charges in a simplified ChargeCoupled Device (CCD);

FIG. 4 illustrates a conventional detector array;

FIG. 5 illustrates a detector array employing graded forward motioncompensation (FMC) according to the present invention (shown in theforward oblique mode);

FIG. 6 illustrates a conventional detector array employingcolumn-segmented forward motion compensation;

FIG. 7 illustrates a projection of a focal plane array onto the groundfor side oblique image collection;

FIG. 8 illustrates the operation of a graded FMC detector arrayoperating in a side oblique viewing mode;

FIG. 9A illustrates the image motion rate and FIG. 9B illustrates theCCD line rate for a graded FMC detector array operating in a sideoblique viewing mode;

FIG. 10 is an illustration of an example environment for theelectro-optical reconnaissance system operating in a side obliqueviewing mode;

FIG. 11 is an illustration of an example environment for theelectro-optical reconnaissance system operating in a forward obliqueviewing mode;

FIGS. 12 and 13 illustrate the operation of a graded FMC detector arrayin a forward oblique mode according to the present invention;

FIG. 14 is an illustration of an example environment for theelectro-optical reconnaissance system operating in a vertical viewingmode;

FIG. 15 illustrates the operation of a graded FMC detector array in avertical mode according to the present invention;

FIG. 16 illustrates a layout of a focal plane array according to thepresent invention;

FIG. 17 illustrates a preferred embodiment of the focal plane array withside bus connections;

FIGS. 18, 18A, 18B, and 19 illustrate the clocking sections of adetector array with a column-segmented imaging area according to analternative embodiment of the present invention;

FIG. 20 illustrates a pixel model for determining a time constant forV-phase gates according to the present invention;

FIG. 21 is a block diagram of the electro-optical reconnaissancesystem's camera electronics according to a preferred embodiment of thepresent invention;

FIG. 22 is a flow chart of the camera motion compensation controlprocess according to the present invention;

FIG. 23 is a block diagram of the timing generator and CCD driveelectronics implemented in the reconnaissance system according to thepresent invention;

FIG. 24 illustrates example frame timing and line timing signalsaccording to the present invention;

FIG. 25 is a block diagram of the shutter exposure control according tothe present invention;

FIG. 26 plots exposure time versus slit width for two example shutterspeeds according to the present invention;

FIG. 27 plots exposure time as a function of solar altitude for variouslenses according to the present invention; and

FIG. 28 is block diagram of the digital preprocessor according to apreferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

1. Overview and Discussion of the Invention

The present invention is directed to a method and system for thecompensation of image motion during aircraft reconnaissance missions. Inparticular an imaging focal plane array (FPA), such as a charge-coupleddevice (CCD) is utilized to record a target scene. In order tocompensate for the forward motion of the aircraft, the charge in theimaging device is transferred across the device. The rate of transfer isuniform across the CCD, but varies in time in accordance with theportion of the target being imaged. This is accomplished by using amoving window (or slit) shutter which scans the projected image of thetarget across the imaging device. The charge transfer rate is variedbased on the position of the projected image on the imaging device. Thepresent invention controls the design and the specification of theoptics, mechanical shutter, and the CCD to construct a low cost andproducible image motion compensation (IMC) system for a very highperformance reconnaissance system. The manner in which this isaccomplished is described in detail below.

To put the invention in context, a brief discussion of some of theproblems associated with current aerial reconnaissance systems will bedescribed. For example, image smear is normally present in severaldifferent aerial reconnaissance mission scenarios. FIG. 1 illustratesthe types of image motions encountered when the camera is lookingdirectly downward (vertical), looking toward one side (side oblique),and looking forward at a selected angle of depression from the horizon(forward oblique). In FIG. 1, the rectangular focal plane array is shownas projected onto the ground. The relative magnitude and direction ofthe image motion within the frame is indicated by the length anddirection of the motion-indicating arrows.

In the vertical example 102, the aircraft 104 flies directly over top ofthe target or scene of interest. Thus, all of the image motion is of asingular direction (opposite to the flight direction) and magnitude.That is, all the parts of the image move together, parallel to the lineof flight. This motion is uniform in magnitude throughout the frame 106.

In the side oblique case 112, the motion remains parallel to the line offlight 111, but is not of the same magnitude throughout the frame.Objects nearest to the flight path, represented by arrow 114, appear tomove fastest. Those objects further away from the line of flight,represented by arrow 116, appear to move more slowly, in proportion totheir distance from the flight path 111.

The forward oblique case 122 is more complex. In this case, the imagemotion is composed of two vectors. The first is parallel to the line offlight as in the examples discussed above. Here again, the magnitude ofthis motion vector varies with range from the aircraft: the motionvector is larger the nearer a given point in the image is to theaircraft. Away from the line of flight, a second motion (of much lowerabsolute magnitude) occurs. As points approach the aircraft, they appearto “fly off” to the side of the format. Points to the left of the flightpath “fly off” to the left, and points to the right of the flight path“fly off” to the right. The magnitude of this vector increases thecloser a given point is to the aircraft.

2. Example Environment

Before describing the invention in detail, it is useful to discussexample reconnaissance techniques in which the invention can beutilized. Preferably, the present invention can be implemented in avariety of electro optical reconnaissance systems. For example, fourbasic types of imagery sensors are: strip mode, pushbroom, panoramicsector scan and framing. The choice of a particular technique depends onthe specific operational need. These four example reconnaissance systemsare illustrated in FIGS. 2A-D.

FIG. 2A illustrates a stip mode sensor. Strip mode sensors create animage by pointing an EO (electro-optical) focal plane at the ground,through a lens, orientated perpendicular to the line of flight. The EOfocal plane device can be a body-fixed sensor or a moveable sensor.Ground geometry is maintained and/or corrected by adjusting the linerate of the focal plane to compensate for image motion. For example, asshown in FIG. 2A, a linear detector array (not shown), located inaircraft 202 is oriented perpendicular to the flight direction. Thearray comprises a line of pixels to create a first dimension of scenecoverage. The forward motion of the aircraft creates a second dimensionof coverage along the flight path of the aircraft.

A second reconnaissance technique is called a “pushbroom” technique andis illustrated in FIG. 2B. Pushbroom sensors are a variation of stripmode sensors, in which the linear instantaneous field of view is movedfore and aft in the in-track or along-track (along the flight path)direction in order to achieve stereo imaging, compensate for imagemotion, and/or create an image “frame.” Once again, as shown in FIG. 2B,a linear array is oriented perpendicular to the flight direction. Thecombination of the forward motion of the aircraft and the fore/aft scanof the array produces an overlapping second dimension of coverage.

A panoramic sector scan sensor technique, illustrated in FIG. 2C,creates a “frame” of imagery by moving the instantaneous field of viewof the focal plane perpendicular (or across-track) to the line offlight. The width of the frame (in degrees) is fixed by the focal lengthof the sensor and the length of the focal plane, and the across-trackcoverage is determined by the scan speed and the duration of the scan ofthe sensor. In this example, a linear detector array is orientedparallel to the flight direction. The scan of the array, e.g., from thehorizon down, produces the second dimension of coverage.

A fourth technique, known as a framing sensor technique, is illustratedin FIG. 2D. Framing sensors “instantaneously” collect area images, muchas a snapshot camera does. For example, an area detector array ispointed to a target, then frames of imagery are collected. Up untilrecently, frame size and resolution were limited, due to the size andfewer number of pixels in staring arrays.

Framing sensors are advantageous for specific applications. For example,a framing sensor is uniquely able to capture forward oblique imagerycontaining the horizon. This capability can give the reconnaissancepilot flexibility when maneuvering the aircraft near the target. Inaddition, a framing camera can provide improvements in low light levelperformance, continuous stereo coverage and reduced image artifactsresulting from low frequency motions.

Until recently, EO framing cameras were not operationally viable due totheir small image area and technology limitations associated with dataprocessing and storage. Improvements in wafer fabrication and imageprocessing technologies now make this type of camera feasible. Asdescribed below, such high resolution framing cameras can complimentother tactical reconnaissance sensors, especially in the forward obliquemode and for stereo imaging.

The present invention is described in terms of this example environment.Description in these terms is provided for convenience only. It is notintended that the invention be limited to application in this exampleenvironment. In fact, after reading the following description, it willbecome apparent to a person skilled in the relevant art how to implementthe invention in alternative environments.

3. Electronic Forward Motion Compensation (FMC)

The use of Charge Coupled Devices (CCDS) in place of film allows for amethod of “electronic” image motion compensation. In this method, theelectronic signal being formed in the detector array by the image can beshifted to move along with the motion of the image falling on the array.This charge transfer concept is illustrated schematically in FIG. 3.This method can be used to reduce smear caused by the image motion whileallowing for longer exposure times.

FIG. 3 shows a simplified CCD detector represented as a single column ofpixels 302. The incident light on each pixel (or potential well), forexample pixel 304, generates free electrons which are collected at thepixel site. By the application of clocking waveforms A, B, and C atinput 306, the charges (electrons) collected in a pixel well are shifteddown the column. If all pixels are clocked together, a “bucket brigade”like transfer of the signals is achieved. According to the presentinvention, the pixels are clocked during the exposure period at a rateequal to the rate of image motion. Thus, the signal generated by aspecific image point will move to stay with that image point. Thismethod of charge transfer eliminates smear due to the image motion whileincreasing the effective exposure time.

There are at least three methods of compensating for image motion whichare electronic in nature; average FMC, graded FMC and segmented FMC. Allof these methods of FMC can be utilized in the present invention. Thesemethods are each described below. Additionally, these methods arecontrasted with an uncompensated imager.

a. No FMC

FIG. 4 illustrates an uncompensated imager. The uncompensated imager isa simple, but very large imager comprising rows and columns of pixels,as well as attendant readout structure (e.g., amplifiers, etc.). In theuncompensated imager no attempt is made to eliminate or minimize theimage-smearing effects of image motion. The film camera used with anuncompensated imager is typically one in which the film is flat andfixed, and the shutter (between-the-lens or focal plane) is simplyopened to produce the desired length of exposure. Whatever image motionsoccur during exposure, together with the associated smear-induced lossof image quality, are simply tolerated.

For example, a detector array 402 based on an uncompensated imagerincludes X columns, each of which has Y pixels. The array is exposed toa moving image and the signal is then shifted out as in a CCD or read bythe application of an X and Y address clock. Image smear causes a lossof image quality as a function of its magnitude.

b. Average FMC

An improvement over the uncompensated imager configuration is an averageFMC imager. In average FMC imagers, as charge is collected duringexposure, the charge is also moved in synchronization with the motion ofthe image which produced it. For example, the IMMIC (Integrating ModeMoving Image Chip) is a known average FMC imager.

An average FMC imager can also be described with respect to FIG. 4. Thedetector array 402 comprises X columns, all of which transfer charge atthe same rate. Each column comprises Y pixels. During an exposure, allcharges can be moved at a rate equal to the average motion rate. Adetector array based on average FMC has one set of clock lines (φ₁, φ₂,φ₃) which transfer charge for the entire array at a rate proportional toclock speed.

Also referred to as Time Delay and Integration (TDI), the average FMCmethod is used to increase the signal-to-noise ratio. Such charge motioncan be implemented using a variant of the simple large chip discussedabove by shifting data along the CCD columns while the shutter is open.The image motion rate for all columns is the same. The image motion rateis selected by a camera system housing the average FMC imager to matchthe average image velocity over the chip during exposure. While averageFMC imagers provide average FMC correction, they do not compensate forthe different magnitudes of the image vectors at different distancesfrom the flight path. Therefore, unless the image motions are uniform,even with a perfect match to the desired average charge/image velocity,certain columns would exhibit lead or lag smear errors. Even thoughthese errors are much less than for the uncompensated imager, theseerrors result in less than optimum performance. In average FMC, either abetween-the-lens or focal plane shutter can be used.

c. Graded FMC

As discussed above, in either side oblique or forward oblique imagingapplications, forward image motions are not the same at all positions inthe field of view (FOV). In order to solve the problem of non-uniformimage motion as a function of position of the object being imaged, it isnecessary to alter the charge motion rate for each column (or group ofcolumns). The present invention is based on graded FMC. Graded FMCimagers use a combination of a time varying charge motion rate and anexposure controlled by a focal plane shutter slit. With graded FMCimagers, the charge motion rate is uniform across the columns of thearray, but it varies in time as a function of the portion of the imagerbeing exposed by the slit at a given instant.

This graded FMC approach utilizes a focal plane shutter so that only aportion of the array is exposed at any one time. Thus, the inventionmatches the charge motion rate with the position of the shutter slit asit traverses the chip. As a result, the optimum charge motion rate canbe selected corresponding to the exposed portion of the array. Becauseonly one area is exposed during the time associated with a given chargetransfer rate, a nominally ideal image motion compensation can beobtained on-chip. Concerning focal plane array chip design, graded FMCimagers are similar to average FMC imagers. However, a reconnaissancesystem based on graded FMC imagers, utilizes the added refinements of afocal plane shutter and column clocks synchronized to the changes inimage motion. For example, FIG. 5 illustrates the graded FMC conceptused in the forward oblique imaging mode.

A detector array utilizing graded FMC has one set of clock lines whichtransfer charge for the entire array at a rate proportional to clockspeed. The array is made up of X columns, all of which transfer chargeat the same rate. Each column is made up of a number of pixels. Pixelsignal is shifted along all columns keeping pace with the rate of imagemotion seen through the slit. Thus, according to the present invention,the moving exposure slit is used to determine the exposure time for anyportion of the array, and to allow tracking of variable image motionacross the array.

d. Segmented FMC

A fourth category of detector chip, referred to as segmented FMC, can bedesigned to work with either a between-the-lens shutter or a focal planeshutter camera. The segmented FMC concept is shown in FIG. 6. Here thearea of the imager is broken up into some number of segments, where eachsegment is a group of columns. The size of the segments will be dictatedby the magnitude of the differential motion from “side” to “side” of thearray and the practicality of adding ever more segments. The average FMCand graded FMC systems represent an example of a single segment. Forexample, a segmented FMC imager having 16-segments requires 16-times thedrives/clocks, etc. of a graded FMC device.

The column segmented detector array is segmented into sections. Eachsection is clocked to move charge at a different rate in order to keeppace with differing image rates. Each segment is made up of a number ofcolumns, all of which transfer charge at the same rate. Each column ismade up of a number of pixels.

For example, an array which can be implemented in conjunction with thesegmented FMC is disclosed in the Lareau '597 patent, incorporatedherein by reference.

According to the present invention, a graded FMC detector array isincorporated into an electro-optical reconnaissance system. Further, thereconnaissance system utilizes a moving shutter or slit to perform imagemotion compensation in the vertical, forward oblique, and side obliquemodes of operation. Using a graded FMC imager approach reduces thepotential complexity to the design and cost of the focal plane array.Alternatively, the present invention can be implemented using acolumn-segmented focal plane array. The manner in which these FMCimagers operate is described in detail below.

4. Side Oblique Operation in Detail

Image motions and operations of the graded FMC imager according to thepresent invention are described in detail for the side oblique mode ofoperation. In the side oblique image collection mode, compensation ofthe image motion is complex. Although image motion remains monotonic indirection, the magnitude of the image motion is a function of theposition of a given column of pixels in the focal plane array (FPA)relative to its position within the format.

The projection of the FPA onto the ground in a side oblique mode ofoperation is illustrated in FIG. 7. The further away the projection of aparticular column of pixels is from the flight track, the more slowlythe scene image traverses it. To correct for this unidirectional, butunequal, image motion throughout the format, alternative means totransfer the charges along each column can be implemented.

For example, one method of achieving FMC is to divide the columns intosub-groups of a few columns each and to transfer charges along eachsub-group at some average rate deemed to be sufficiently approximate forthat sub-group. This column-segmented FMC method is described above.Each sub-group requires a separate set of adjustable clocks. Such a setof clocks is calculated to work at the best compromise for a given V/Hand depression angle combination. In addition to adding greatly to thecomplexity of the chip drive electronics, the added complexity of thedetector chip makes it difficult, expensive, and risky to produce. Thesesame factors tend to limit the ultimate size (i.e., pixel count) of acolumn-segmented imaging device.

An alternative approach, according to a preferred embodiment of thepresent invention, is to combine the charge motion handling techniquedescribed above with the incorporation of a focal plane shutter withinthe camera. This combination achieves graded FMC.

For example, recall the FPA projection illustrated in FIG. 7. Accordingto the present invention, a focal plane shutter having an exposure slitwhich runs parallel to the columns of CCD pixels is added to thearrangement. This shutter traverses the array from left to right,sequentially exposing different columns of pixels to the target scene.

The operation of a FPA with graded FMC is illustrated in FIG. 8. Forexample, at the far point of the frame 802, the magnitude of the imagemotion is at its smallest, as illustrated by arrow 803. At the mid framepoint 804, the magnitude of the image motion has increased. At the nearpoint of the frame 806, the magnitude of the image motion is at itsgreatest, as this corresponds to the objects closest to the aircraft.The rate at which charge is swept down the columns is varied uniformlyin magnitude across the entire FPA. Without a focal plane shutter inplace, this charge motion rate might be correct for one column or smallgroup of columns, but would be incorrect for all other column groups.However, by making the charge transfer rate (for the entire FPA)consistent at any one instant but variable with the position of theexposure slit in the focal plane shutter, the charge transfer rate isvaried such that the charge transfer rate is matched to the image motionrate in that particular column in the center of the exposure slit. Thisprinciple is illustrated in FIG. 9A, which shows how the image motionrates vary as a function of distance. Thus, only the image ratecorresponding to the position of the slit is “seen” by the array at anygiven time.

The CCD line rate is made to vary as a function of slit position tomatch the image motion rate, as shown in FIG. 9B. At the conclusion ofslit travel, the complete array is read out at a maximum rate. Thecharge motion in the unexposed areas is not matched to the correct imagemotion rate, but this is of no consequence since no imaging light isbeing collected outside of the exposure slit area. As illustrated inFIG. 8, for side oblique operation, the focal plane shutter (or slit) isoriented parallel to the charge transfer columns and is movedperpendicular to the image motions.

In the side oblique mode of operation, the graded FMC imager approachprovides FMC without the added design and charge transfer complexitiesof the segmented FMC approach. In particular, these risks aresignificantly reduced for large scale FPAs, such as a 9216 pixel by 9216pixel CCD.

To understand the rate at which charge is transferred in order toperform FMC, it is beneficial to examine the geometry of the focal planearray and the target. FIG. 10 illustrates the side oblique geometry. Inthis geometry:

F=Focal length

H=Altitude (ALT)

V=Aircraft velocity

φ=in track angle

θ=cross track angle

γ=depression angle

The position at which a point of the target is focused on the focalplane (i.e., the CCD) as a result of the lens is given by theRectilinear Lens Image Transfer Relation. In the side oblique mode, thederivative of the image transfer relation determines the motion in thex-direction (i.e., V_(CCD) in the x-direction). For an ideal lens, theimage transfer relation is:

x=F(tanφ)

Note that for non-ideal lenses, this transfer relation changes,depending on the imperfections in the lens (i.e., x=F(tanφ+k₁φ³+k₂φ⁵)).

The point where φ=0 indicates the center of the area of the target“seen” by the lens. Values of φ other than zero indicate a point on thetarget separated from the center point in the in-track direction by thatangle.

The rate of change of distance x across the focal plane with respect tothe in-track motion is given by:$\frac{x}{\varphi} = \frac{F}{\cos^{2}\varphi}$

The rate of change in the in-track direction as a function of time isdictated by the target geometry and shown by the relationship:$\frac{\varphi}{t} = {\frac{V\quad \cos \quad \varphi}{\frac{H}{\sin \quad \gamma \quad \cos \quad \varphi}} = {\frac{V}{H}\sin \quad \gamma \quad \cos^{2}\varphi}}$

The rate of motion of a point of the target across the CCD (dx/dt) isthe product of the motion across the focal plane and the change in thein-track direction. The charge transfer velocity is always in track(along the direction of flight). Thus the charge transfer velocity,V_(CCD), (in-track, i.e., perpendicular to the principle plane or alongline B from FIG. 10) is determined by:$V_{CCD} = {\frac{x}{t} = {{\frac{x}{\varphi} \cdot \frac{\varphi}{t}} = {{\frac{VF}{H}\frac{\sin \quad \gamma \quad \cos^{2}\varphi}{\cos^{2}\varphi}} = {\frac{VF}{H}\sin \quad \gamma}}}}$

Thus, V_(CCD) is independent of φ when θ=0.

In order to determine the charge transfer velocity variation along theprinciple plane, the tangential effects must be examined instead ofimage transfer effects. The effective focal length is given by:$F_{EFF} = \frac{F}{\cos \quad \theta}$

The change in position on the image sensor as a function of the changein the cross-track direction is given by:$\frac{x}{\varphi} = \frac{F}{\cos \quad \theta}$

Similar to the case with the in-track direction (i.e., along line B),the velocity V_(CCD) of the image along line P (of FIG. 10) is given by:$V_{CCD} = {\frac{x}{t} = {{\frac{x}{\varphi} \cdot \frac{\varphi}{t}} = {{{\frac{F}{\cos \quad \theta} \cdot \frac{V}{H}}\sin \quad \left( {\gamma \pm \theta} \right)} = {\frac{FV}{H\quad}\quad \frac{\quad {\sin \left( {\gamma \pm \theta} \right)}}{{\cos \quad \theta}\quad}}}}}$

Where:$\frac{\varphi}{t} = \frac{V}{\frac{H}{\sin \left( {\gamma \pm \theta} \right)}}$

These equations describe the image motions that are compensated forimaging in the side oblique mode of operation. These equations can beused to form a look-up table that is utilized in the camera controlprocessing system described below in Section 7 (b) (i).

5. Forward Oblique Operation in Detail

Image motions and operations of the example FMC methods are nowdescribed in detail for forward oblique look angles. According to thepresent invention, forward oblique motions are compensated by using afocal plane shutter for a graded FMC imager. The advantages in using thegraded FMC approach are described below by way of a comparison to thepreviously discussed FMC approaches.

The forward oblique geometry is illustrated schematically in FIG. 11. Inthe forward oblique mode of image collection, the center line of the FPAis aligned to the direction of flight just as in the vertical case. Now,however, the optical axis of the camera is pointed upward from Nadir(i.e., the point directly below the aircraft).

For a FPA operating in the forward oblique mode, as shown in FIG. 12,the columns of pixels 1202 run from the “top” to the “bottom” of the FPA1203. The magnitude of charge motion in the forward oblique orientationvaries with position along the Y—Y axis of FIG. 12. All columns share acommon value for image and charge velocities for any given point alongthe Y—Y axis, whereas all column velocities are common along the X—Xaxis.

The variation in apparent in-track (along a column) image motion issimilar, but not equal, for all the rows. Image motion varies from beingslower at the “top” and being faster at the “bottom”. This image motionis compensated by using a graded FMC approach.

By way of comparison, in forward oblique operation, average FMC areaimagers (oriented with their columns parallel to the Y—Y axis of FIG.12) move charge along all columns at the same rate. Preferably, the rateis selected to be an average value of image motion and correctlycompensates only at one point along the Y—Y axis.

According to the present invention, graded FMC imagers (with theircolumns aligned parallel to the Y—Y axis) operate in the forward obliquemode in the same manner as in the side oblique case with one exception:the array (chip) is rotated 90° with respect to the direction ofexposure slit travel. This method of application is illustrated indetail in FIG. 13. A focal plane shutter traverses the image area fromtop to bottom, and the charge motion rate is varied as a function of theposition of that shutter along the Y—Y axis. For example, in FIG. 13A,position 1302 corresponds to the position of the exposure slit at thefar point of the frame, position 1303 corresponds to the position of theexposure slit at the mid point of the frame, and position 1304corresponds to the position of the exposure slit at the near point ofthe frame. Here, the slit is oriented perpendicular to the chargetransfer columns and is moved parallel to the vector of image motion. Inaddition, as shown in FIG. 13B, the forward image motion rates vary as afunction of V/H. As in the side oblique case described above, only theimage rate at the position of the slit is “seen” by the array at anygiven time. The CCD line rate is made to vary as a function of slitposition to match the image motion rate, as shown in FIG. 13C. At theconclusion of slit travel, the complete array is read out at a maximumrate. Thus, according to the present invention, ideal matching of imagemotion and charge motion can be maintained.

Column-segmented imagers can also perform image motion compensation inthe forward oblique mode of operation by utilizing a moving focal planeshutter as discussed above. In particular, each column segment isclocked at the same rate because there is essentially no differentialmotion across the row. However, the complexities of multiple verticalclocks and potentially low yield CCD architecture are still present.

The equations describing the forward oblique in-track image motion,V_(CCD), are derived in a similar manner as described above in Section4. Referring back to FIG. 11, in this geometry:

F=Focal length

H=Altitude (ALT)

V=Aircraft velocity

φ=cross track angle

θ=in track angle

γ=depression angle

The image transfer relationship of a lens determines the y-velocity ofthe image on the focal plane and is determined by:

y=F(tanθ)

The image motion on the focal plane as a function of the offset angle isgiven by: $\frac{y}{\theta} = \frac{F}{\cos^{2}\theta}$

The target geometry provides the rate of change of the image Line ofSight (LOS) to target as follows:$\frac{\theta}{t} = {\frac{V\quad {\sin \left( {\gamma \pm \theta} \right)}}{\frac{H}{\sin \left( {\gamma \pm \theta} \right)}} = {\frac{V}{H}{\sin^{2}\left( {\gamma \pm \theta} \right)}}}$

Therefore, the velocity across the focal plane (in-track)of the image isgiven by:$V_{CCD} = {\frac{y}{t} = {{\frac{y}{\theta} \cdot \frac{\theta}{t}} = {\frac{FV}{H}\quad \frac{\quad {\sin^{2}\left( {\gamma \pm \theta} \right)}}{\cos^{2}\theta}}}}$

For values perpendicular to the principle plane (where θ=0), theeffective focal length F_(EFF) is given by:$F_{EFF} = \frac{F}{\cos \quad \varphi}$

Therefore, the change in the position of a point y on the focal plane isgiven by: $\frac{y}{\theta} = \frac{F}{\cos \quad \varphi}$

The target geometry provides:$\frac{\theta}{t} = {\frac{{V \cdot \quad \sin}\quad \gamma}{\frac{H}{\sin \quad {\gamma \cdot \cos}\quad \varphi}} = {\frac{V}{H}\sin^{2}{\gamma cos}\quad \varphi}}$

Therefore, the velocity of the imaged point across the focal plane is$V_{CCD} = {{\frac{y}{\theta} \cdot \frac{\theta}{t}} = {\frac{VF}{H}\sin^{2}\gamma}}$

Note that, as the above equation shows, for the forward oblique casethere is no dependence on cross-track angle φ when θ=0. As in the sideoblique case, these image motion equations can be used to form a look-uptable that is utilized in the camera control processing system describedbelow in Section 7 (b) (i).

6. Downward Looking (Vertical) Operation in Detail

A third method of operation for the graded FMC imager according to thepresent invention is in the straight downward looking (vertical)orientation. This is illustrated schematically in FIG. 14. In this modeof operation, the columns of the FPA 1402 are oriented to flow from “topto bottom” of the perceived frame of imagery. The rate of motion is thesame at all points of the FOV for an undistorted lens looking perfectlyvertically.

According to the present invention, graded FMC imagers, oriented withthe columns parallel to the Y—Y axis, move charge along all rows at thesame rate. Image motion is fully compensated since it is uniform.Neither a between-the-lens nor focal plane shutter is required toachieve FMC, although either type of shutter can be used.

The operation of a graded FMC detector array in the vertical orientationis shown schematically in FIG. 15. Since the graded FMC imager of thepresent invention is already equipped with a focal plane shutter for theside and front modes, that shutter can be used for the vertical mode ofoperation. Since image motion is uniform, the charge transfer rateremains fixed throughout the shutter scan time. The orientation of theshutter with respect to the transfer columns is optional because it isnot necessary to limit exposure to a specific column as a function ofimage motion rate. This flexibility makes it convenient to move thecamera from side oblique to downward looking without the need to rotatethe chip with respect to the slit. Similarly, a camera initiallyoriented to operate in the forward oblique mode can be rotated down fordownward looking operation without the need to re-orient the chip withrespect to the slit.

In the preferred embodiment of the present invention, the focal planeshutter can be oriented such that the transparent slit traverses eitherside-to-side, or top-to-bottom (the orientation of which is illustratedin FIG. 15). For the vertical orientation, the magnitude of chargemotion is constant with the position of the exposure slit along the Y—Yaxis and is fixed for a given row along the X—X axis.

7. Preferred Embodiment of the Present Invention

The present invention can be incorporated in numerous differentreconnaissance systems using current and yet-to-be-developed cameras,focal planes, and electronics systems adapted to provide a chargetransfer rate that is uniform across the CCD and is time-varying incoordination with the focal plane shutter motion. The present inventionis designed to utilize a variety of possible focal plane arrays, CCDimaging electronics, and system electronics to meet a specific set ofdesired performance specifications and parameters of the operatingenvironment (e.g., ambient light conditions, aircraft velocity,altitude, distance to target, etc.). It will be apparent to one skilledin the art that alternative embodiments and structures may be utilizedto meet these specifications and parameters. Additionally, these oralternative embodiments and/or structures may be utilized to meetalternative specifications and/or parameters.

a. Focal Plane Array

Although the invention can be utilized with numerous different focalplane array configurations, a preferred focal plane array configurationin this example operating environment is provided below. After readingthis description, it will become apparent to those skilled in the arthow to implement the invention using alternative focal plane arrays.

i. Focal Plane Array Size

Focal plane array size is driven by performance requirements andapplication parameters. Preferably, a detector array is large enough tomeet the application's field of view (FOV) requirement and to achievethe desired performance (such as that defined by the National ImageryInterpretation Rating Scale (NIIRS)) from a specified altitude. Forexample, a high quality reconnaissance system can produce a NIIRS indexof approximately 8. In this example operating environment, a GeneralImage Quality Equation (GIQE) is used with an estimate of the GroundSampled Distance (GSD) to produce this high NIIRS index value. For thisfirst order analysis, it is assumed that no image enhancement is used, asystem modulation transfer function (MTF) of 15% is achieved at Nyquistand the typical GIQE signal-to-noise ratio, for an ƒ/4 optical systemusing a typical detector array at 20° solar altitude, is about 23:1.Applying desired light level and contrasts, this results in a GSD ofabout 2.4 inches to produce NIIRS 8 performance. These above mentionedstandards are known to those of skill in the reconnaissance art.

In the example operational embodiment, the required cross-track field ofview from a 500 foot altitude is 115°, which produces cross trackcoverage of 1570 feet. If this coverage is resolved uniformly,approximately 7850 pixels to sample at 2.4 inches per pixel are requiredto achieve NIIRS 8 throughout the field of view (FOV). To achieve adesired coverage of 140° (2747 feet), approximately 13,737 pixels areneeded to sample the FOV uniformly at NIIRS 8. In the along-trackdirection, the required field of view is 75° (767 feet). This coverageis sampled to NIIRS 8 with approximately 3836 pixels. Therefore, apreferred imager has between approximately 7850 and 13,737 pixels in thecross-track direction and at least 3836 pixels in the along-trackdirection. For example, an imager with the performance equivalent to a100 megapixel framing camera ±20% requires between approximately 9000 by9000 pixels and 11,000 by 11,000 pixels, or another appropriatemultiple. Thus a large scale, monolithic CCD is the preferred focalplane array according to the present invention.

Alternatively, the present invention can also utilize a step-stareimager, which is known in the relevant art. The line of sight of theimager can be repositioned by either moving the lens assembly or bymoving a mirror/prism in front of the lens. However step-stareapproaches introduce an added level of mechanical complexity to areconnaissance system. In addition, increased coverage can be achievedby mechanically butting two arrays together, eliminating the problemsassociated with step-staring. However, the added cost of matching fourchips, processing complexity, and the loss at the critical centralregion due to butting creates an undesirable tradeoff for a framingcamera.

The focal plane arrays described above are provided for example only.The above example illustrates the manner in which the array size ischosen for a particular set of performance specifications andapplication criteria. For other applications or performancespecifications, alternative focal plane array sizes can be implementedas would be apparent to one of ordinary skill in the art.

ii. Array Architecture

Eliminating complexity in the device design and processing is essentialto obtaining a sufficient yield to make an imager economically viable.The preferred array architecture for the rows in the main format area isthe conventional three-phase structure, which is known to bestraightforward to process with high yields. The column structuredepends on the type of on-chip forward motion compensation (FMC). Thepreferred embodiment of the present invention utilizes a graded (i.e.,non-segmented) FPA for use in a system based on the graded FMC approachdescribed above.

The functional layout for a 9216 pixel by 9216 pixel device according toone embodiment is shown in FIG. 16. According to a preferred embodimentof the present invention, the full-frame CCD imager has an 8.1centimeter (cm) by 8.1 cm main format area 1602 containing an array of9216×9216 pixels. Each pixel size is approximately 8.75 micrometers(μm)×8.75 μm. The serial register 1604 at the bottom of main format 1602has four detector/amplifier outputs 1610-1613. The sampling rate foreach output is approximately 25 megapixels/second. A greater or lessernumber of amplifier outputs can be utilized depending on the readoutrequirements.

As described above in connection with FIG. 3 and the description ofconventional CCD operation in section 3, during the integration orexposure period, an electronic representation of an image is formed whenincident photons create free electrons that are collected within theindividual photosites. These photoelectrons are collected locally by thebias action of the three “V” electrodes 1606 and the column boundariesformed by the P+ channel-stop implants. These column boundaries areillustrated as channel stops 1705 in FIG. 17. FIG. 17 also illustratesthat in a preferred embodiment, poly V-phase gates 1706 with side busconnections are utilized.

After an integration time, a shutter (such as a focal plane shutter or abetween the lens shutter described above) closes to block illuminationon the focal plane and the readout cycle begins. During readout, thecomplete image is shifted out by changing the potentials on electrodesV₁, V₂, and V₃ in a sequence which causes packets of signal charge tomove line by line into the horizontal output register.

Referring back to FIG. 16, during each line readout time period, thevoltage on the electrodes comprising the horizontal shift registers arechanged or “clocked” to shift pixel charges into the output detector andamplifier structure (1610-1613). One-by-one the charge packets aredumped on a small conductive area called the floating diffusion (FD).There the charge packets change the FD potential by an amount equal tonq/C, where n is the number of electrons/packet, q is the electroncharge in coulombs and C is the FD capacitance. The FD voltage is sensedand buffered to the signal output by an on-chip FET source followerstructure located within a detector/amplifier, such as amplifier 1610.

When FMC is required, the normally static bias condition for the “V”electrode voltages are modified to cause charge packets to transfer at arate corresponding to the rate of image motion normal to the linedirection of the array matrix. The FMC charge shift is always in thesame direction. The rate of charge shift can vary as the slit opening inthe focal plane shutter moves from top to bottom of the CCD format. TheFMC line shifts that occur during the exposure period are small innumber compared to the total lines of the CCD format.

In one embodiment, an approach to multiport operation is to separateonly the horizontal output register 1604 into segments, where eachsegment contains an output detector/amplifier structure. A uniquefeature of this output register design is a taper region between thelast active format line and throughput register. This eliminates any gapbetween columns of the active format.

Alternatively, a column-segmented CCD, having as many as 16 segments canbe utilized in order to achieve sufficient FMC to produce good qualityimages. For example, FIG. 18 depicts the architecture of acolumn-segmented CCD array 1802 having N column segments. If the numberN of column segments is 16, these 16 column segments thus require anincrease in the number of separate variable V clocks from 3 to 48, withan associated increase of the off-chip drive electronics. Additionally,as shown in insert 1804, a column-segmented design requires metalizationin the imaging area which significantly reduces CCD yield. Moreover, acolumn-segmented CCD requires an increased number of contact holes (thelocations where the metal makes contact to the underlying structures),as shown in insert 1806. As discussed below, this added complexity isrequired in order to vertically clock the column-segmented focal planearray.

iii. Vertical Clocking

FIG. 19 further illustrates a portion of a column-segmented CCD shownabove in insert 1806 of FIG. 18. Note that array 1902 includes metalstraps 1904 over the corresponding channel stops with thru-holemetal-poly contacts, such as contact hole 1906. For very large areaarrays, side bussed polysilicon gate lines as illustrated in FIG. 17 aremuch easier to process with high yields than the more complex metalstrapped structure shown in FIG. 19. Metal strapping, which is usuallydone to achieve very fast V clocking, is a required feature forcolumn-segmented arrays.

The yield limitations of metal strapping arise from the need to makesmall diameter openings in the insulating dielectric coatings such thatthe metal straps make contact with each of the polysilicon gate lines(see e.g., contact hole 1906). As shown in FIG. 19, for a three phaseCCD, every row of N pixels contains N/3 contact regions (one every thirdpixel). In small pixel devices (<12 μm²) with ½ to 1 μm overlap of thephase gates, there is very little room in each of the three gates toetch down and contact the first poly layer. This is further aggravatedby alignment inaccuracies between φ₁, φ₂, φ₃ and the contact layer.

The contact problems are exacerbated when the array image section issegmented, such as in array 1802 from FIG. 18. To minimize loss ofinformation, the gap between segments must be kept small: yet the metalover the channel stop must be kept from causing an electrical shortbetween adjacent segments of the same phase.

As mentioned above, a graded FMC imager having a side bussed polysilicongate structure without metal strapping or format segmentation, such asthe array structure illustrated in FIG. 17, is preferred for full-frameimager production. According to a preferred embodiment of the presentinvention, for the 9216 by 9216 array (totaling approximately 85megapixels) operating at an output of 100 megapixels per second, thefull format can be read out in 85/100 or 0.85 seconds. The correspondingline shift time is 0.85/9216 or 92.2 microseconds (μs), which is themaximum time allowed for clocking each line to the output serialregister.

Burst clocking, as is the case for TV cameras where the line is onlyshifted during horizontal blanking, can require even shorter shift time.Although some feedthrough of clock into the video does occur, thissignal contamination is line coherent and readily removed with a digitalstored compensating signal. FIG. 20 shows a model used to determine thevertical poly line time constant for the preferred 9216 pixel×9216 pixelCCD. A time constant (T) value of 11.3 microseconds (μs) is calculatedbased on the pixel resistance (Rpix) and pixel capacitance (Cpix) valueslisted on the right hand side of model 2002. This T value fully supportsclocking with a preferred 92.2 μs line time interval.

iv. CCD Imager

Characteristics for the CCD imager in a preferred embodiment of thepresent invention are listed below in Table 1. Other CCD's with othercharacteristics can be used as would be apparent to those of skill inthe art.

TABLE 1 FULL-FRAME IMAGE SENSOR SPECIFICATIONS Active Pixels per line9216 Active lines (progressive readout) 9216 Pixel size, μm  8.75 × 8.75Image format, mm 80.64 × 80.64 Number of output registers 4 (on oneside) Number of outputs 4 Data rate 100 MHz Resolution: MTF at Nyquist50% Q Saturation (100% pixels) 70 k electrons RMS noise electrons 18Dynamic range 72 dB Pixel random nonuniformity 3% Dark current (20° C.,1 second) <480 electrons Fixed pattern noise (20° C., 1 second)  <75electrons QE, 550 nm 29%    650 nm 44%    750 nm 35%    850 nm 20%Number of clocks (vertical) 3 Number of overscan columns 1/segmentNumber of black reference columns 2 × 20 (20L × 20R) Number of blackreference lines 20 (bottom) Clock amplitude (vertical) 10 V Total numberof lines 9236 Number of clocks (horizontal) 2 Clock amplitude(horizontal) 10 V Conversion factor, μV/electron 3 Linearity 99% Pixelrate per output 25 MHz

b. System Electronics

A block diagram of an example system electronics architecture isillustrated in FIG. 21. In this example architecture, the camera backelectronics 2100 comprise an imaging section 2104 and an electronicsunit 2106. According to this example architecture, the imaging section2104 includes imaging electronics 2108 comprising an analog processor2110, thermoelectric (TE) cooler controller 2116, shutter exposurecontrol 2114 and the FPA (or CCD) drive electronics 2112. Theseelectronics are used to command and communicate with the focal planearray 2123, in conjunction with the FMC methods discussed above. Asdescribed above, a lens 2120 collects the target image onto FPA 2123. Afocal plane shutter 2121 traverses across FPA 2123 at a ratecorresponding to image motion of the objects viewed in the scene. Therate at which shutter 2121 traverses FPA 2123, as well as the slit widthof shutter 2121 are determined based on the commands of imagingelectronics 2108. The TE cooler controller 2116 controls a TE cooler2124, which maintains the operating temperature of FPA 2123. The cameracontrol electronics also include a power supply module 2145.

The camera back electronics 2100 also include an electronics unit 2106to ultimately process the image of the target scene as viewed by FPA2123. The electronics unit 2106 includes the camera host processor (orCPU) 2140, two digital preprocessors 2130 and 2131, the data compressionelectronics 2134, the tape recorder interface 2138, and a DCRSI 240recorder 2139. The digital preprocessors 2130 and 2131 utilize ASICtechnology. In addition, the camera CPU 2140 controls the CCD clockspeed and its variation to implement FMC. The functionality of theseindividual components is discussed below in detail. Except where notedbelow, these electronics can be conventional electronics that are knownin the art. Alternative architectures can be implemented to performthese functions, as would be apparent to one of skill in the art.

i. Camera Control Process

The electronics illustrated in FIG. 21 are used to perform FMC accordingto the present invention. An exemplary FMC method utilizing theseelectronics, and based on the image motion equations described above insections 4 and 5 is shown in FIG. 22.

FIG. 22 is a flow diagram that describes the camera control processaccording to on e embodiment. The control of light level involvesdecision points and simple look-up tables in steps 2206 and 2208.Referring to both FIGS. 21 and 22, the process starts at the start of aframe at step 2202, where the input light level to the camera ismeasured by a light sensor, such as light sensor 2122. This light levelis compared to a standard light level at step 2204. For example, thechosen standard light level is 277 foot candles, which corresponds to asolar altitude of approximately 3° above the horizon. At high solaraltitudes (>277 foot candles), the decision is made to use the PrimaryExposure Time Look-up Table to determine exposure time (step 2208). Forlonger exposure times (>2 milliseconds (ms)) required for low ambientlighting conditions, the shutter speed is slowed down to approximately50 inches/second in order to keep the slit width narrow enough forgraded FMC. For short exposure times (≦2 ms), a shutter speed ofapproximately 300 inches/second is selected.

For the lowest ambient light conditions, an exposure time is determinedfrom the Low Light Exposure Time Look-up Table (step 2206). This tableutilizes the instantaneous values of aircraft velocity, altitude, andcamera look angle at the start of each frame in addition to the lightlevel. One example of when the low light exposure time look-up table isused is when the measured light level is <277 foot candles, such as whenthe solar altitude is less than 3°. Other thresholds can be defined forthe lowest ambient light condition based on mission requirements.

The output of the exposure time look-up steps 2206 or 2208 is theoptimal exposure time and selection of a shutter speed. Thecorresponding slit width is determined in step 2210, where the slitwidth chosen is a product of the exposure time and the shutter speed.Correspondingly, the exposure time can be determined by dividing theslit width by the shutter speed.

Once exposure time is determined, for each frame, the CCD clockingprofile is calculated in step 2212 to accomplish FMC. In one embodiment,this profile is determined by the host processor 2140 (in step 2212). Instep 2212 a look-up table based on the in-track image motion rateequations described above in sections 4 and 5 (depending on the obliquemode of operation) is used with the following inputs: exposure time;aircraft velocity, V; aircraft altitude, H; depression angle of camera(fixed for flight); camera installation location (fixed for flight);shutter trigger pulse; and focal length. In a preferred embodiment, theprocess is re-initiated at the start of each camera frame. The resultingFMC clocking signal is sent to imaging electronics 2108 to perform FMC(step 2214).

ii. CCD Drive Electronics

The CCD drive electronics (such as CCD drive electronics 2112 from FIG.21) comprise two essential parts, a timing generator, and a clock drivestage. These elements are shown in detail in FIG. 23, which representsan exemplary design to perform optimum CCD and system clocking. Thetiming generator is responsible for two functions, CCD readout andForward Motion Compensation (FMC).

As shown in FIG. 23, a 150 MHz master clock signal is divided by six (atlocation 2302) to provide the local 25 MHz pixel clock, from which allCCD clocks and digital controls are derived. The horizontal counter 2304provides a time base for pixel counting operations, which includedefining the vertical shift interval at FPA vertical clock generator2310 and clocking of the horizontal output CCD registers at FPAhorizontal clock generator 2311. The vertical counter 2306 likewiseprovides a time base in the vertical direction of the CCD.Alternatively, higher frequency clocks may also be utilized to providefor greater smoothness of steps to the vertical clocks.

Multi-tap delay lines 2314 a-b are employed on the horizontal andvertical clocks to permit minute refinements in phase relationships,allowing optimization of vertical and horizontal charge transferefficiency.

Additionally, the 25 MHz clock is buffered and skew-compensated toprovide synchronous timing to both the video sampling analog-to-digitalconverters (ADC) and the subsequent digital preprocessing.

Synchronization signals are generated at frame synchronization unit2319, in the form of frame and line syncs 2320 a-b, respectively. Thesesync signals synchronize the digital preprocessors 2130 and 2131 (inFIG. 21) to the quantized video stream.

FIG. 24 represents example line and frame timing output pulses 2404 and2402, respectively. For the example operating conditions describedabove, the line and frame timing are derived as follows:

Line Timing

9216 pixels/line÷4 segments=2304 pixels per segment

2304 active pixels+10 pre/post scan=2314 pixels/line

2314 pixels÷25 MHz+10 μs (vertical clock interval)=102.6 μs/line=9747lines/second

Frame Timing

9216 active lines+20 pre/post scan=9236 lines/frame

Readout time=9236×102.6 μs=0.9472 second

Maximum exposure time (at 50 ips)=0.0638 second

Frame time=0.9472+0.0638=1.011 second/frame=0.989 frames/second.

iii. Forward Motion Compensation (FMC)

As explained above in section 3, during the CCD exposure interval, thecharge pattern formed in the CCD corresponding to the optical image isshifted at the rate the image is moving in order to compensate for theeffect of the aircraft's forward motion. This charge pattern movement isaccomplished by applying variable clock rate vertical transfer signalsto the CCD during the exposure time. Referring back to FIG. 21, thesesignals are generated in the clock waveform generators of CCD driveelectronics 2112, and are controlled in both frequency and duration byCPU 2140. The V/H signal is interpreted by the host processor 2140 toprovide rate-based CCD vertical shift commands (seen in FIG. 22, CCDclock control 2214) to a timing generator (explained in detail above),in turn commanding the CCD to shift for motion compensation. FMC occursduring the integration time of the CCD. At this time, all CCD clocks arein an inactive state until commanded by the processor to perform avertical shift for FMC.

The vertical clock drivers move the charge through the integrationsites, and into the horizontal output register (see FIG. 16). Accordingto a preferred embodiment, the vertical clock drivers 2310 supply a 10volt peak-to-peak drive waveform into 4 nano-fared (nf) gate capacity. Atypical CCD readout rate according to the present invention isapproximately 9747 Hz. However, during FMC, the vertical transfer can goas high as 12 KHz. For example, in a preferred embodiment, a knownMIC4451 driver (manufactured by MICREL Semiconductor, Inc., of San Jose,Calif.) can be used as the vertical driver 2310. Other known drivers canalso be utilized based upon cost and performance considerations.

The horizontal clock drivers 2311 move the charge through the horizontalregister to the floating diffusion (FD) section (of amplifiers 1610-1613in FIG. 16) where the output voltage signal is formed. These horizontaldrivers supply up to a 10 volt peak-to-peak waveform into 125 pico-fared(pf) at the 25 MHz pixel rate. A discrete component circuit known in theart can be utilized as no satisfactory monolithic circuit drivers arecurrently available. The horizontal and vertical drivers each haveadjustable offset and gain capabilities, to permit tuning to optimalperformance for each individual array.

iv. Shutter Exposure Control

In a preferred embodiment, CCD exposure is controlled by two focal planefunctions: the width of the focal plane shutter exposure slit and thespeed of the exposure slit (e.g., shutter 2121 illustrated in FIG. 21).In alternative embodiments, exposure can be controlled by eitherfunction alone. As mentioned above, in a preferred embodiment, the widthof the slit is approximately 0.1 inch to 0.5 inches, and the speed ofthe exposure slit is approximately 300 inches per second for shorterexposure times and 50 inches per second for longer exposure times. Thus,the speed of the exposure slit can be constant across the FPA or varied,depending upon the necessary forward motion compensation required.

In the illustrated embodiment, the existing light sensor output (such asfrom light sensor 2122 in FIG. 21) is digitized by the shutter exposurecontrol circuitry 2114. An example exposure control block diagram isshown in FIG. 25. The light sensor signal 2502 is converted to look-uptable values located in look-up table 2508 after buffering (at buffer2504) and digitization (at ADC 2506) to separately drive the speed 2510and slit width 2512 of the shutter. An example exposure control profileis shown in FIG. 26, where exposure time is plotted as a function ofslit width. An example plot of optimized exposure times for the CCD, instandard daylight conditions, is a function of the sun angle asrepresented in FIG. 27. It should be noted that illumination levelschange dramatically at dawn and dusk conditions. Also, the stepindicated by an asterisk (*) in FIG. 27 is due to a filter inserted infront of the imager.

As described above in connection with FIG. 22, the CCD's exposure time(t), which is the time it takes the slit to pass any single pixel, isgiven by:

t=w/s

where w=slit width in inches, and s=shutter speed in inches/second. In apreferred implementation, the exposure time is set by the incidentlight. Because the final signal amplitude is controlled by an automaticgain control (AGC) function in the digital preprocessing sections 2130and 2131, this open loop control function, which mimics that used on thefilm camera, represents the preferred approach.

For example, for light levels down to 3° solar altitude (i. e., the 277foot candles level), the exposure time follow the curves shown in FIG.27. Below these light levels (essentially at dusk), the length ofexposure is limited by the calculated image motion variations across thechip. Camera focal length, V/H and depression angle are used to selectproper look-up table exposures at the very low light levels aspreviously shown in FIG. 22, step 2206.

v. Digital Preprocessor

In a preferred embodiment, as shown in detail in FIG. 28, digitalpreprocessing is performed on two identical Circuit Card Assemblies(CCAs) 2802 and 2804. CCA 2802 processes inputs from channels 1 and 2,and CCA 2804 processes inputs from channels 3 and 4. These CCAsrespectively correspond to digital preprocessors 2130 and 2131,illustrated in block diagram form in FIG. 21.

Because CCA 2802 and CCA 2804 are similar, only the elements comprisingCCA 2802 are described. In one embodiment, CCD pixel data is stored inhigh speed Static Random Access Memory (SRAM) configured as FirstIn/First Out (FIFO) memory (see location 2810). This FIFO memoryoperates as line buffers to facilitate replacing defective pixels withnearest neighbor processing. Defective pixels are identified duringlaboratory testing and characterization of the CCD. Locations of thesedefective pixels are stored as (X,Y) coordinates in Programmable ReadOnly Memories (PROM) 2815 on the digital preprocessor board. Theselocations are compared to the (X,Y) coordinates of the FPA as it is readout. When a match occurs, the defective pixel is replaced by a knownnearest neighbor processing routine. This implementation reduces thehardware complexity required for defective pixel correction for the 9216pixel×9216 pixel FPA.

In a preferred embodiment, memory addressing is generated by a FieldProgrammable Gate Array (FPGA) based timing and address generator 2814,which runs synchronously and in tandem with the CCD timing generator2812 on the EO module CCA 2802. CCD timing generator 2812, which issynchronized by the frame and line sync 2809, uses the 25 MHz videosampling clock. This synchronous operation eliminates any possibility ofinjecting uncorrelated noise into the video.

The memories 2815 are read out into the Application Specific IntegratedCircuit (ASIC) 2820. The 12-bit video data at full scale is equivalentto saturation of the CCD. For most operational scenarios, the videosensor signal occupies only a fraction of an ADC's 12-bit dynamic range.Specular reflections, manifest as high intensity transients, aresubtracted out with a digital low pass filter within ASIC 2820. Haze,which manifests itself as a DC level (i e., no counts in the lower binsof the gray scale histogram), is also subtracted out at ASIC 2820. TheAutomatic Gain Control (AGC) functionality in ASIC 2820 detect themaximum and minimum amplitude of the signals, maintaining a runningaverage over multiple lines. The AGC gain is then adjusted to take fulladvantage of the 8-bit dynamic range. This 12 to 8-bit conversioneliminates the non-essential video information while preserving theactual imagery data. Further, the 8-bit data is in the proper format forthe image bus control ASIC 2824 and the data compressor 2825. The AGCaction optimizes sensor performance and reduces the raw data rate by 30%without degradation of the original image.

Illumination (vignetting) correction is performed at chip 2822 byapplying correction coefficients (e.g., for the 1″, 3″ and 12″ lenses)to the gain input of AGC ASIC 2820. During factory calibration, curvesof the illumination roll-off across the FPA are established. The inverseof these curves is programmed into Programmable Read Only Memory (PROM)2815, which provides these gain corrections to the video.

As noted, the electronic components described above can be conventionalelectronics that are known in the art. Alternative architectures can beimplemented to perform the aforementioned functions, as would beapparent to one of skill in the art.

4. Conclusion

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. Thus, the breadth and scope of thepresent invention should not be limited by any of the above-describedexemplary embodiments, but should be defined only in accordance with thefollowing claims and their equivalents.

What is claimed is:
 1. An electro-optical reconnaissance system,comprising: a focal plane array including a main format area having aplurality of photo-sensitive cells arranged in rows and columns, whereinsaid focal plane array is configured to detect a projected image of ascene and to convert said image into an electronic charge representationof said image; and a shutter having a controllable exposure slitproximate to said focal plane array, wherein said exposure slit is movedacross said focal plane array to define areas of exposure, each area ofexposure having an associated image motion that is substantially uniformacross said area of exposure, wherein said electronic chargesrepresenting said image are transferred at a charge transfer ratecorresponding to said image motion in said associated area of exposureexposed by said shutter exposure slit, as said exposure slit is movedacross said focal plane array.
 2. The electro-optical reconnaissancesystem of claim 1, further comprising: a camera control electronics unitdriving said plurality of photo-sensitive cells with a clocking signalfor an exposed portion of said focal plane array corresponding to saidcharge transfer rate, wherein said clocking signal corresponds to aposition of said exposure slit and said image motion.
 3. Theelectro-optical reconnaissance system of claim 2, wherein a width and aspeed of said exposure slit are adjustable, and wherein said cameracontrol electronics unit controls said exposure slit width.
 4. Theelectro-optical reconnaissance system of claim 3, wherein said focalplane array is a charge coupled device (CCD).
 5. The electro-opticalreconnaissance system of claim 4, wherein said focal plane array furthercomprises: a horizontal output register having a predetermined number ofsegments, wherein each of said segments includes an outputdetector/amplifier structure.
 6. The electro-optical reconnaissancesystem of claim 2, wherein the reconnaissance system is installed in avehicle capable of moving in a forward direction, and wherein saidcamera control electronics unit comprises: an imaging electronicssection comprising an analog processor to process said transferredelectronic charges representing said image, focal plane array (FPA)drive electronics to generate said clocking signal to drive said FPA, ashutter exposure control unit to control shutter parameters, whereinsaid shutter parameters include said exposure slit width and said speedof said exposure slit; a signal processing electronics unit comprising adigital preprocessor coupled to said FPA drive electronics and to saidanalog processor, to receive and further process said electronic chargerepresentation of said image, and to provide a digital processed imagesignal; and a camera central processing unit (CPU), to process missionparameter inputs and provide processed mission parameter information tosaid imaging section to perform forward motion compensation (FMC) ofsaid image.
 7. The electro-optical reconnaissance system of claim 6further comprising: a lens to focus said scene onto said focal planearray; signal recording means coupled to an output of said signalcompression means to record a forward motion corrected image of saidscene; and a power supply to provide power for said camera controlelectronics unit.
 8. The electro-optical reconnaissance system of claim7, further comprising a light sensor in communication with said shuttercontrol unit; and a thermo-electric cooler to control an operatingtemperature of said focal plane array.
 9. The electro-opticalreconnaissance system of claim 8, wherein said focal plane array ismounted on an adjustable mount coupled to said vehicle, wherein theelectro-optical reconnaissance system performs forward motioncompensation in a forward oblique mode of operation, a side oblique modeof operation, and a vertical mode of operation.
 10. The electro-opticalreconnaissance system of claim 6, wherein said shutter control unitcomprises: a buffer to receive a signal generated by said light sensorindicating lighting conditions of the scene; an analog to digitalconverter coupled to said buffer to convert said light sensor signalinto a digital signal; and a look-up table to convert said digitizedsignal into a look up table value to drive said shutter, wherein saidlook-up table provides drive signals corresponding to said exposure slitspeed and said exposure slit width.
 11. The electro-opticalreconnaissance system of claim 6, wherein said FPA control electronicscomprise: a timing generator to generate a master timing signal and toprovide for focal plane array readout and FMC, wherein said mastertiming signal is divided by a predetermined value to provide a localtiming signal; a horizontal counter to provide a time base for pixelcounting operations; a vertical counter to provide a time base in thevertical direction of said focal plane array; a horizontal clockgenerator coupled to said horizontal and vertical counters, to provide ahorizontal clocking signal to said focal plane array; a vertical clockgenerator coupled to said horizontal and vertical counters, to provide avertical clocking signal to said focal plane array; and a framesynchronization unit, coupled to said horizontal and vertical counters,to generate frame sync signals and line sync signals.
 12. Theelectro-optical reconnaissance system of claim 11, wherein said FPAcontrol electronics further comprise: a plurality of multi-tap delaylines to define a phase relationship of said horizontal and verticalclocking signals.
 13. The electro-optical reconnaissance system of claim11, wherein said digital preprocessor comprises: a circuit card assembly(CCA) to process inputs from said imaging electronics section.
 14. Theelectro-optical reconnaissance system of claim 13, wherein said CCAcomprises: a Static Random Access Memory (SRAM) configured as FirstIn/First Out (FIFO) memory to store pixel data from said focal planearray, wherein said FIFO memory facilitates replacing defective pixelswith nearest neighbor processing; a timing generator coupled to saidframe sync and line sync signals; a Field Programmable Gate Array (FPGA)address generator coupled to said timing generator to generate memoryaddressing; a Programmable Read Only Memory (PROM) coupled to said FPGAaddress generator to store locations of said defective pixels; anAutomatic Gain Control (AGC) ASIC to reduce said pixel data withoutdegradation of the original image, wherein said pixel data is reducedfrom twelve-bit form to eight-bit form; an illumination chip to correctfor vignetting effects of said image; and an image bus coupled to saidAGC ASIC to receive said eight-bit data format.
 15. The electro-opticalreconnaissance system of claim 14, wherein said AGC ASIC includes meansto subtract out specular reflections contained on said image, subtractout haze contributions contained on said image, and maintain a runningaverage of said image data.
 16. An electro-optical reconnaissance systemfor performing forward motion compensation, wherein said reconnaissancesystem is installed in a vehicle capable of moving in a forwarddirection, comprising: a focal plane array including a main format areahaving a plurality of photo-sensitive cells arranged in rows andcolumns, wherein said focal plane array is configured to detect aprojected image of a scene and to convert said image into an electroniccharge representation of said image, and wherein said focal plane arrayis oriented to view said scene in a forward oblique mode of operation;and a focal plane shutter, having a controllable exposure slit proximateto said focal plane array, wherein said exposure slit is moved acrosssaid focal plane array to define areas of exposure, each area ofexposure having an associated image motion that is substantially uniformacross said area of exposure, wherein said exposure slit is orientedparallel to a direction of said rows, and wherein said electroniccharges representing said image are transferred at a charge transferrate corresponding to said image motion in said associated area ofexposure exposed by said shutter exposure slit, as said exposure slit ismoved across said focal plane array.
 17. The electro-opticalreconnaissance system of claim 16, further comprising: a lens to focussaid scene onto said focal plane array; and a camera control electronicsunit driving said plurality of photo-sensitive cells with a clockingsignal for an exposed portion of said focal plane array corresponding tosaid charge transfer rate, wherein a width and a speed of said exposureslit are adjustable, wherein said camera control electronics unitcontrols said exposure slit width, wherein said clocking signalcorresponds to a position of said exposure slit and said speed of saidexposure slit, and wherein said clocking signal corresponds to a rate ofmotion of objects contained in a portion of said scene viewed by saidfocal plane array.
 18. The electro-optical reconnaissance system ofclaim 17, wherein said clocking signal is generated in accordance withan in-track image motion, wherein said in-track image motion isdetermined by$\frac{FV}{ALT}\frac{\sin^{2}\left( {\gamma \pm \theta} \right)}{\cos^{2}\theta}\quad {and}\quad \frac{V\quad F}{ALT}\quad \sin^{2}\gamma$

where F=Focal length, ALT=Altitude of the vehicle, V=velocity, θ=intrack angle, and γ=depression angle.
 19. The electro-opticalreconnaissance system of claim 16, wherein said focal plane array is acolumn-segmented charge coupled device (CCD).
 20. An electro-opticalreconnaissance system for performing forward motion compensation,wherein said reconnaissance system is installed in a vehicle capable ofmoving in a forward direction, comprising: a focal plane array includinga main format area having a plurality of photo-sensitive cells arrangedin rows and columns, wherein said focal plane array is configured todetect a projected image of a scene and to convert said image into anelectronic charge representation of said image, and wherein said focalplane array is oriented to view said scene in a side oblique mode ofoperation; and a focal plane shutter, having a controllable exposureslit proximate to said focal plane array, wherein said exposure slit ismoved across said focal plane array to define areas of exposure, eacharea of exposure having an associated image motion that is substantiallyuniform across said area of exposure, wherein said exposure slit isoriented parallel to a direction of said columns, and wherein saidelectronic charges representing said image are transferred at a chargetransfer rate corresponding to said image motion in said associated areaof exposure exposed by said shutter exposure slit, as said exposure slitis moved across said focal plane array.
 21. The electro-opticalreconnaissance system of claim 20, further comprising: a lens to focussaid scene onto said focal plane array; and a camera control electronicsunit driving said plurality of photo-sensitive cells with a clockingsignal for an exposed portion of said focal plane array corresponding tosaid charge transfer rate, wherein a width and a speed of said exposureslit are adjustable, wherein said camera control electronics unitcontrols said exposure slit width, wherein said clocking signalcorresponds to a position of said exposure slit and to said image motionin said area of said scene exposed by said shutter exposure slit. 22.The electro-optical reconnaissance system of claim 21, wherein saidclocking signal is generated in accordance with an in-track imagemotion, wherein said in-track image motion is determined by$\frac{V\quad F}{ALT}\quad \sin \quad \gamma \quad {and}\quad \frac{FV}{ALT}\frac{\sin \left( {\gamma \pm \theta} \right)}{\cos \quad \theta}$

where F=Focal length, ALT=Altitude, V=velocity, φ=in track angle,θ=cross track angle, and γ=depression angle.
 23. The electro-opticalreconnaissance system of claim 20, wherein the focal plane array is acolumn-segmented charge coupled device (CCD).
 24. A method for providingforward motion compensation (FMC) for an electro-optical reconnaissancesystem in a vehicle capable of forward motion, said opticalreconnaissance system including a moveable shutter exposure slit and afocal plane array configured to detect a projected image of a scene andto convert said image into an electronic charge representation of saidimage, comprising the steps of: (1) moving the shutter exposure slitacross the focal plane array to define areas of exposure, each area ofexposure having an associated image motion that is substantially uniformacross said area of exposure; (2) transferring electronic chargesrepresenting the image at a charge transfer rate corresponding to saidimage motion in said associated area of exposure exposed by said shutterexposure slit, as said exposure slit is moved across said focal planearray; (3) a measuring a light level of a scene to be imaged by thereconnaissance system; (4) comparing the measured light level to apredetermined light level value; (5) determining an exposure time bycomparing the measured light level to an exposure time look-up table,(6) determining a forward motion compensation profile corresponding tothe exposure time and mission parameter inputs; and (7) sending a signalcorresponding to said forward motion compensation profile to anelectronics unit of the electro-optical reconnaissance system to performFMC.
 25. The method of claim 24, wherein step (3) comprises the stepsof: (a) determining an exposure time by comparing the measured lightlevel to a primary exposure time look-up table, if the measured lightlevel is greater than the predetermined light level value; and (b)determining an exposure time by comparing the measured light level to alow light exposure time look-up table, if the measured light level isless than the predetermined light level value.
 26. The method accordingto claim 24, wherein step 3(a) further comprises the step of: sending ashutter speed signal corresponding to the determined exposure time to ashutter exposure control unit, wherein a faster shutter speedcorresponds to shorter exposure times, and wherein a slower shutterspeed corresponds to longer exposure times.
 27. The method according toclaim 24, wherein step 3(b) further comprises the step of: utilizing aset of instantaneous mission parameters to determine the exposure time,wherein the set of instantaneous mission parameters includes at leastone of aircraft velocity, altitude, and camera look angle.
 28. Themethod according to claim 24, further comprising the step of: (6)determining a exposure slit width for the exposure slit corresponding toproduct of the exposure time and the exposure slit speed.
 29. The methodaccording to claim 24, wherein the forward motion compensation profiledetermined in step (4) corresponds to a look-up table value, whereinsaid look up table value is calculated based on in-track image motionrate equations, and wherein the in-track image motion rate equationsutilize a set of mission parameter inputs that include: aircraftvelocity, V; aircraft altitude, H; depression angle of camera (fixed forflight); camera installation location (fixed for flight); shuttertrigger pulse; and focal length.
 30. The method according to claim 29,wherein the electro-optical reconnaissance system is operating in a sideoblique mode of operation, wherein the in-track image motion isdetermined by$\frac{V\quad F}{ALT}\quad \sin \quad \gamma \quad {and}\quad \frac{FV}{ALT}\frac{\sin \left( {\gamma \pm \theta} \right)}{\cos \quad \theta}$

where F=Focal length, ALT=Altitude, V=Aircraft velocity, φ=in trackangle, θ=cross track angle, and γ=depression angle.
 31. The methodaccording to claim 29, wherein the electro-optical reconnaissance systemis operating in a forward oblique mode of operation, wherein thein-track image motion is determined by$\frac{FV}{ALT}\frac{\sin^{2}\left( {\gamma \pm \theta} \right)}{\cos^{2}\theta}\quad {and}\quad \frac{V\quad F}{ALT}\quad \sin^{2}\gamma$

where F=Focal length, ALT=Altitude, V=Aircraft velocity, θ=in trackangle, and γ=depression angle.
 32. An electro-optical reconnaissancesystem, comprising: a focal plane array including a main format areahaving a plurality of photo-sensitive cells arranged in rows andcolumns, wherein said focal plane array is configured to detect aprojected image of a scene and to convert said image into an electroniccharge representation of said image; a shutter having a controllableexposure slit proximate to said focal plane array; means for moving saidexposure slit across said focal plane array to define areas of exposure,each area of exposure having an associated image motion that issubstantially uniform across said area of exposure, and means fortransferring said electronic charges representing said image at a chargetransfer rate corresponding to said image motion in said associated areaof exposure exposed by said shutter exposure slit, as said exposure slitis moved across said focal plane array.